Exercise can even grow the brains of older adults, says researcher Kirk I. Erickson, PhD, assistant professor of psychology at the University of Pittsburgh.

In his recent study, older adults without Alzheimer's, aged 60 to 80, who walked moderately for 30 to 45 minutes three days a week for a year had a 2% increase in the volume of their hippocampus, a region of the brain important for memory.

He also found growth in another brain area important for memory, the prefrontal cortex.

"I would say this is pretty dramatic," he tells WebMD. "This is only after one year of exercise and moderate intensity at that."

He presented his findings at the Alzheimer's Association International Conference 2012 in Vancouver.

Another study found benefits for resistance training in adults who already have noticeable changes in thinking and memory. Until now, it has been little studied for its effect on memory and brain health.

About 5.4 million Americans have Alzheimer's disease, the Alzheimer's Association estimates.

Exercise and Alzheimer's Risk: Studies

Erickson assigned 120 older adults who had no dementia but had been inactive for the past six months to either a moderate-intensity walking group or a stretching-toning group for a year.

Before the study, he did brain imaging studies to measure the size of the hippocampus. He also looked at the size of the prefrontal cortex. "The prefrontal cortex is also involved in some memory functions, and it also tends to decay as we get older," he says.

He took blood samples and measured the concentrations of a substance called brain-derived neurotrophic factor (BDNF). "BDNF is critical in the development of new neurons," Erickson tells WebMD. "It also seems to be critical in learning and memory."

He gave the men and women a battery of tests to assess their thinking skills. He looked at such measures as their ability to switch between doing different tasks and their memory.

He found the 2% increase in hippocampus size. "Generally in this age range, people are losing 1% to 3% a year of hippocampal volume," he says. The changes in the size of the hippocampus were correlated with changes in the blood levels of the BDNF.

He also found that higher levels of fitness were linked with a greater size of the prefrontal cortex.

His advice? "Get up off the couch." Exercise, he says, is ''one of the most promising nonpharmacological treatments to improve brain health."

Resistance Training and Alzheimer's Risk: Study Details

Resistance training may help people who already have a condition known as mild cognitive impairment (MCI) delay the onset of dementia, says Teresa Liu-Ambrose, PhD, PT, assistant professor of physical therapy at the Vancouver Coastal Health Research Institute and University of British Columbia.

MCI boosts the risk of getting dementia, Liu-Ambrose says. "More than half will convert to a diagnosis of dementia within five years," she says.

Her goal was to see if resistance training or other exercise might stall that decline.

In her study, she assigned 86 women, aged 70 to 80, who had MCI to one of three groups:

Twice-a-week resistance training for an hour each session.

Twice-a-week aerobic training for an hour each session.

Toning and balance training for an hour each session.

She tested them before and after on such mental skills as attention, working memory, and everyday problem solving.

For instance, she says, they would look at the word "blue" printed in yellow ink. They would be told to call out the ink color rather than read out the word.

In real life, this skill might translate to remembering you need to take a different route to a friend's house because you plan to stop at the dry cleaner, she says.

She tested associative memory, too, which can become a problem with age. If you have trouble with associative memory, for instance, you may remember someone told you about a great sale at your favorite store, she says. "But you can't remember who told you."

Compared with the balance and tone group, those who did resistance training improved their performance on the test of attention and other tests and on the associative memory test.

The aerobic group had improvement in balance and mobility and heart health measures.

Resistance training may be a good workout for those who already have cognitive decline, Liu-Ambrose says.

Exercise and Brain Health: Perspective

Mark Gluck, PhD, director of the Rutgers University Memory Disorders Project, reviewed the new research findings for WebMD.

"These important studies remind us that the pathway to successful aging and brain health lies not just in the hands of the pharmaceutical companies, but in our own two feet," he says.

Based on the new findings, which form of exercise would he recommend to help brain health?

"There is more support, so far, for the role of aerobic exercise in supporting brain function than there is for resistance training," he says. "Aerobic exercise also improves sleep and reduces stress, both of which are important for maintaining brain health and a sharp mind.''

These findings were presented at a medical conference. They should be considered preliminary, as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.